Extraction of a predetermined component from a composition comprising the predetermined component with organic solvents is conventional. For example, caffeine is extracted from coffee beans with chlorinated hydrocarbons such as methylene chloride to produce decaffeinated coffee, and soybean oil is extracted from soybeans with hexane. In all these methods, residual amounts of the solvent must be recovered from the extracted solids to limit solvent make-up costs.
Methods have also been disclosed for solvent extraction of bitumen from bitumen containing tar sands. Typically, these methods involve comminuting the tar sand, contacting the tar sand with about 2 to about 10 parts of solvent per part of bitumen present in the tar sand, and separating a bitumen solvent phase from spent tar sand. However, the residual solvent is difficult to remove from the spent tar sands because the solvent is strongly adsorbed.
The residual solvent is generally recovered from spent tar sands by heating to evaporate the solvent. Heating tar sands tailings in indirect surface exchangers such as evaporative dryers is expensive because of solids handling difficulties, dust problems, expensive heat transfer surface area, high heating demands to remove the last trace amounts, and poor heat recovery. Thus, lower cost processes for solvent recovery in bitumen extraction are important.
U.S. Pat. No. 3,475,318, "Selective Solvent Extraction Plus Filtration of Tar Sands", Gable, issued Oct. 28, 1969, discloses solvent recovery from spent tar sands by steam stripping. In one embodiment the spent sands are placed on a filter and steam is pushed or pulled through the spent sand by a pressure drop across the filter bed, thereby stripping off the solvent. In another embodiment steam is passed through the spent tar sands on the filter until just prior to steam breakthrough at the bottom of the filter bed. The partially steamed sand is then passed into a rotary kiln dryer where solvent is stripped from the sand with steam entering the dryer in a direction essentially countercurrent to the sand. Gable does not disclose steam stripping in a packed column.
Steam stripping has also been used to remove ethanol from fermentation mashes containing suspended fine solids. In this method the fermentation mass slurry is countercurrently contacted with steam in a stripping column with disk-donut trays. However, the fermentation slurries have solids concentrations less than 10 weight % and the ethanol is dissolved in the slurry fluid rather than adsorbed on the fine solids.
None of the disclosed methods of solvent stripping from spent tar sand tailings, however, employ treating a slurry of the spent sand. None of the disclosed methods strip the solvent from the spent sand. It is therefore an object of the invention to strip residual solvent from spent tar sands in a packed column. It is a further object to achieve substantially complete solvent removal from spent tar sand. Yet another object is an energy efficient solvent stripping method capable of operating at high flow rates. Other objects appear below.